The invention relates to a machine tool with an interchangeable bit for the machining of profiled bearing surfaces, and for example valve seats of internal-combustion engines.
Currently, such machining is carried out by a special machine equipped with a motorized spindle in which a tool holder is mounted. The tool holder 1, shown in the appended FIG. 1, is equipped on the one hand with an axial guide pin 2 for positioning it in the valve guide 3 of a cylinder head 4 and, on the other hand, with a bit holder 5 comprising two branches 5a which can be seen in FIG. 2. The latter are mounted so as to slide in the tool holder with the possibility of locking in the latter by screws 6. An arm 5b, approximately parallel to the axis of rotation of the spindle, carries a removable, multi-edged bit 7 used for machining a seat 8. FIG. 3 shows that, in current tool holders, the bit 7 has a wedge-shaped heel 10 and is fastened, by a screw 9, to a plane face 12 which, as shown in FIG. 2 is parallel to a diametral plane Pd of the tool holder.
The cutting profile of the bit 7 is matched to the shape of the seat that has to be machined and is defined by at least five parameters, namely, and as shown in FIG. 3:
the angle a of the main cutting edge 13 used for machining the conical bearing surface 8a on which the valve will bear, and therefore constituting the actual valve seat;
the width b of this seat;
the angle c of the cutting edge 14 for the upper clearance 8b of the seat 8;
the width d of this edge; and
the angle e of the cutting edge 15 for the lower clearance 8c of the seat.
Because of the diversity of valve seats, there are thousands of bit types so that they are customized from blanks, for example made of sintered tungsten carbide, the profiling of which is carried out as required. This profiling also comprises the forming of the clearance angle f, shown in FIG. 2.
To allow the cutting edges of these bits to be subsequently reground, without having to use a special grinding machine, the two large faces, namely the front face 7a and back face 7b respectively, of the bits are plane and parallel. This allows the cutting edges to be ground by grinding the front face 7a. Finally, in order to prevent this grinding from moving the cutting edges outside the diametral plane Pd, it is common practice for the bearing face 12 of the bit holder to be positioned so that the cutting edges of a new bit are short of this diametral plane, that is to say are in front of the latter, by an amount of about 0.5 to 0.8 mm, as shown by the amount g in FIG. 2.
This method of renovation, which has hitherto proved satisfactory, reaches its limits because, on the one hand, of the increase in hardness of the seats and, on the other hand, of the impossibility of using the high cutting speeds allowed by carbide bits.
This is because, in order to better withstand the additives in current fuels, the valve seats are manufactured from increasingly harder alloys which are difficult to machine, such as alloys based on carbon, chromium, vanadium, nickel, silicon and manganese.
Moreover, the surface hardness of the seats is increased by the peening of the seat by the valve and by the increase in exhaust gas temperatures. This increase in hardness cannot be compensated for by an increase in cutting speed, as is known in other fields, for the following reasons:
renovation of the seats is carried out dry and the guide pin which rotates in the valve guide, with a small clearance, runs the risk of seizing if it is driven at high speeds;
the structure carrying the tool is not designed for these speeds and, in particular, lacks rigidity, especially when the cutting forces are high because of the length of the cutting profiles.
This lack of rigidity derives from the vertical height of the bit holder with respect to the bearings for guiding the motorized spindle, but also from the mounting of the bit on the bit holder and from the mounting of the bit holder on the tool holder, the effects of the latter being greater due to the radial offset of the bit holder with respect to the axis of rotation of the spindle.
FIG. 3 shows that, in the current arrangements, the resultant Rc of the reactions to the cutting forces being exerted on the various cutting edges 13, 14 and 15 is very often outside the point 17a where the heel of the bit bears on the bit holder. If, as shown in this figure, the manufacturing tolerances, of the angles of the heel and of the angles of the faces of the bit holder serving for supporting the heel respectively, are at their limiting values, that is to say are under unfavorable conditions, the heel bears only on two points 17a, 17b and may allow the bit to pivot in the direction of the arrow 18, due to the couple applied to it by the resultant Rc. This consequently modifies the position of the cutting edges, impairing precision, but also encourages vibration and impairment of the surface finish of the machined faces. It should be noted that this pivoting can occur all the more readily as the back of the bit bears on a plane face.
DE-A-4 124,646 describes a machine tool, with an interchangeable tool for machining profiled parts, in which each tool, on the one hand, carries two opposed bits bonded onto the latter and, on the other hand, has a straight heel and mutually parallel bearing faces parallel to the longitudinal axis of the bit, these faces defining, in cross section, a xe2x80x9cVxe2x80x9d-shaped back profile complementary to that produced in a cradle of the tool holder against which said tool is fastened by a screw passing through it.
This way of mounting the tool in the tool holder improves the rigidity of their connection but complicates the regrinding of the bits bonded onto the tool. Moreover, with this tool profile, and whether the regrinding operation is carried out on the cutting face or on the flank, this operation automatically involves having to modify the cutting profile, and therefore the profile of the seat machined.
The object of the present invention is to remedy these drawbacks by providing a machine tool in which the positioning and the fastening of the bit to the bit holder increase the stiffness and guarantee the geometrical positioning of the cutting profile, both after the first grinding and after regrinding, and therefore guarantee the precision with which the profiled part is machined, while still allowing the same bit to be reground several times by the usual, simple, inexpensive means available in any mechanical machining workshop.
For this purpose, in the machine tool according to the invention, the cradle of the bit holder is in the form of a rectangular trihedron and is inclined transversely by rotation about an axis corresponding to a generatrix of the conical bearing surface of the seat that has to be produced, while the cutting edges of the bit are produced from a blank having three bearing faces forming a rectangular trihedron, namely the back face, a longitudinal face and the rear face, the last two being as-sintered, and the other longitudinal face and the forward face of which, remaining in the as-produced state, are flanks with respect to the front face, the main cutting edge being produced so that its mid-point coincides with a reference point on the axis of inclination of the cradle and lies on this axis at a constant distance from the vertex of the trihedron of the cradle.
With this arrangement, when the bit is positioned in its cradle and fastened by a through-screw, the bearing of its three bearing faces against the faces of the trihedron of the cradle, under the action of the horizontal components of the clamping force of the screw, ensures that the bit and its cutting edge are perfectly positioned.
By virtue of the longitudinal face of the cradle which extends over almost the entire length of the bit, the resultant of the reactions to the cutting forces always lies between the bearing regions of the bit and therefore does not tend to make it pivot about its fastening, thereby guaranteeing that the cutting edges are correctly positioned.
Furthermore, the inclination of the cradle has the advantage, while using bits with parallel, respectively front and back, faces, and therefore easily regrindable bits, of giving the main cutting edge and inclination providing a grinding rake, without thereby modifying the position in space of this preferred edge, and therefore the geometry of the machining of the corresponding bearing surface.
It will be noted that, although the other, nonpreferred, cutting edges have a position in space which is modified, because of their pivoting about the reference edge and not about their own edge, these modifications are not of the type to exceed the manufacturing tolerances, which are generally wider, assigned to the bearing surfaces that they are intended to machine.
The fact of giving a grinding rake, by transversely inclining the cradle, makes it possible to reduce the cutting forces and consequently the reaction forces affecting the rigidity of the bit holder and of the tool holder, but also and above all that of the spindle quill with respect to the spindle bearings.
Finally, the precise positioning, right from the machining for adapting the blank, of the mid-point of the main cutting edge makes it possible to guarantee the precision of the machining by the bit, while reducing the number of bit-holder models, the latter then not being differentiated except by the angle of inclination, with respect to the axis of rotation of the tool holder, of the generatrix of the main bearing surface of the seat that has to be machined.
In one embodiment of the invention, the cradle is provided, in at least the two lower edges of its trihedron, with a clearance groove while the lower edges of the trihedron of the bit are broken by an as-sintered chamfer.
This arrangement makes it possible, despite there being no machining on the longitudinal and rear bearing faces of the bit, to guarantee that this bit is correctly positioned by the cradle.
The invention also relates to a multi-edged interchangeable cutting bit for the machining of valve seats, in which the cutting edges are produced, on a case by case basis by grinding, from a universal blank, for example made of sintered tungsten carbide, preground on its two parallel large faces, namely the back and front faces respectively, and having a through-bore for the passage of a countersunk fastening screw.
According to the invention, the cutting edges are produced from a blank which has, seen from above, the shape of a rectangular trapezoid and, in a region which, bounded between two as-sintered faces, and as flanks, namely the longitudinal and forward faces respectively, is diagonally opposite a rectangular positioning trihedron formed by a longitudinal face, a rear face and the back face.